1. Field of the Invention
The present invention relates to liquid crystal display devices and a method for manufacturing the same and, more particularly, to a liquid crystal display device with improved luminescence and a method for manufacturing the same.
2. Discussion of the Related Art
With progress towards an advanced information society, there is a strong need for development of high quality flat panel display devices with excellent characteristics such as thinness, lightweight and low power consumption. Among those, a liquid crystal display device with superior resolution, color display, image quality, etc. is widely used in various applications including a notebook type computer, a laptop monitor, and the like.
In general, a liquid crystal display device has a structure wherein two substrates have two sides facing each other, each of which has an electrode thereon, and a liquid crystal material is introduced between the substrates. Therefore, when a certain voltage is applied to both of the electrodes to generate an electric field, liquid crystal molecules become movable by the electric field to vary light transmittance so that the liquid crystal display device may display images by the varied light transmittance.
A bottom substrate of the liquid crystal display device is fabricated by using an array substrate having a thin film transistor, which applies signals to a pixel electrode, so as to form a thin layer, lithographically etching the formed thin layer, and repeating these processes. A top substrate of the liquid crystal display device comprises a common electrode and a color filter, which has three colors of red (R), green (G) and blue (B) arranged in sequence, and this top substrate is fabricated by pigment dispersion, dyeing, electro-deposition, and so forth. Among these, the pigment dispersion has superior precision and excellent reproduction, thus being widely applicable.
Such a liquid crystal display device is normally fabricated by forming an array substrate and a color filter substrate, and arranging a pixel electrode on a bottom substrate to correspond to a color filter on a top substrate. During the arrangement step, misalignment problems may occur to cause failures such as light leakage.
In order to solve problems described above, the top substrate may have a wider black matrix and, in this case, a degree of opening of the liquid crystal display device may be reduced.
Therefore, a method has recently been proposed to form a color filter on an array substrate in order to prevent misalignment and improve degree of opening of the liquid crystal display device. Such a structure of a color filter formed on a top side of a thin film transistor refers to a Color Filter on Thin Film Transistor (COT) structure.
FIG. 1 is a cross-sectional view illustrating a typical liquid crystal display device having a COT structure.
Referred to FIG. 1, a gate electrode 12 made of a conductive substance such as metal is formed on a transparent first substrate 11, and a gate insulating film 13 consisting of a silicon nitride (SiNx) or silicon oxide (SiO2) film covers the gate substrate 12.
On the gate insulating film 13 formed on the top side of the gate electrode 12, an active layer 14 made of amorphous silicon may be formed, followed by additionally forming an ohmic contact layer 15, which comprises amorphous silicon and is doped with foreign materials (or impurities), on the active layer 14.
A source electrode 16a made of conductive substance such as metal as well as a drain electrode 16b are formed on a top side of the ohmic contact layer 15, wherein the source and drain electrodes 16a and 16b are used to fabricate a thin film transistor T together with the gate electrode 12.
Although not illustrated in the drawings, the gate electrode 12 is connected to a gate wiring while the source electrode 16a is connected to a data wiring. Both the gate wiring and the data wiring cross to each other at right angles to define a pixel region.
A first substrate 11 including the source and drain electrode 16a and 16b may have a protective film 17 which comprises a silicon nitride film, a silicon oxide film or an organic insulating film in order to protect the thin film transistor T.
In the pixel region on the top side of the protective film 17, a color filter 18 is formed wherein R, G and B colors are aligned in sequence and each color corresponds to each pixel region. The color filter 18 may include a contact hole 19 exposing the drain electrode 16b in addition to the protective film 17.
A pixel electrode 20 made of a transparent conductive substance is formed on a top side of the color filter 18 to be electrically connected to the drain electrode 16b through the contact hole 19.
Further, a second transparent substrate 21 is located a certain distance above the first substrate 11 and a black matrix 22 is placed on an inner side of the second substrate 21 at a position corresponding to the thin transistor T. Although not illustrated, the black matrix 22 has an opening at a position corresponding to the pixel electrode 20 and is formed on a bottom surface of the substrate.
Therefore, the black matrix 22 may prevent light leakage since liquid molecules are tilted on other parts except the pixel electrode 20, and may shield light incident on a channel part, thereby inhibiting generation of light leakage current.
In addition, an over-coat layer 23 is entirely formed on the bottom surface of the second substrate 21 having the black matrix 22.
A liquid crystal layer 30 is formed between the first substrate 11 and the second substrate 21.
As for the liquid crystal display device with a COT structure described above, the color filter 18 is formed on the first substrate 11 so that misalignment of the color filter and the pixel electrode 20 does not occur when the first substrate 11 is combined with the second substrate 21.
Therefore, alignment margin of the black matrix 22 in the second substrate 21 may be reduced and, if a black matrix substance with light penetration inhibitory effects is used to form a barrier pattern, the black matrix 22 on the second substrate 22 may be omitted, thereby improving opening degree of the liquid crystal display device.
FIGS. 2A to 2D are cross-sectional views illustrating a conventional method for manufacturing a liquid crystal display device having a COT structure.
As illustrated in FIG. 2A, a metal substance is deposited on a first transparent substrate 31 and is selectively removed through photolithography so as to form a gate electrode 32 and a common wiring 33.
A gate wiring (not shown) which is connected to the gate electrode 32 and extends in one direction may be formed during formation of the gate electrode.
Continuously, an insulating substance such as a silicon nitride film or a silicon oxide film is thoroughly deposited on a top surface of the first substrate 31 including the gate electrode 32 so as to form a gate insulating film 34.
As illustrated in FIG. 2B, an amorphous silicon layer and another amorphous silicon layer doped with impurities are doped on the gate insulating film 34 in this order.
After that, the amorphous silicon layer doped with impurities and the other amorphous silicon layer located under the doped silicon layer are selectively removed to form an active layer 35 and an ohmic contact layer 36.
Following this, a metal substance is entirely deposited on the first substrate 31 and selectively removed through photolithography to form a source electrode 37a and a drain electrode 37b. 
While forming the source electrode 37a and the drain electrode 37b, a data wiring (not shown) which is extended from the source electrode 37a and crosses the gate wiring at right angles to define a pixel region may also be formed.
The ohmic contact layers 36 exposed by the source electrode 37a and the drain electrode 37b are selectively removed. Herein, the source electrode 37a and the drain electrode 37b may be formed a certain distance apart from each other in order to form a channel in a following process.
Subsequently, a first passivation layer 38 is entirely formed on the top surface of the first substrate 31.
As illustrated in FIG. 2C, a photosensitive material is applied to a top side of the first passivation layer 38, followed by exposing and patterning the same to form a color filter layer 39 in the pixel region.
Since the color filter layer 39 normally comprises R, G and B colors, applying, exposing and developing processes may be repeated three times so as to produce the color filter layer capable of embodying the colors, respectively.
After that, a second passivation layer 40 is entirely formed on the top surface of the first substrate 31 having the color filter layer 39, and then, the second and the first passivation layers 40 and 38 are selectively removed by a photolithographic process, so as to form a contact hole through which the drain electrode 37b is partially exposed.
Next, a transparent conductive material is deposited on the entire portion of the top surface of the first substrate 31 having the contact hole, and is selectively removed by a photolithographic process so that a pixel electrode 41 connected to the drain electrode 37b through the contact hole and a common electrode 42 spaced from the pixel electrode 41 at a certain distance may be formed.
As illustrated in FIG. 2D, a black matrix 52 which is arranged at a certain interval corresponding to a thin film transistor except the pixel region may be formed on a bottom surface of a second substrate 51 corresponding to the first substrate 31, and then, an over-coat layer 53 may be entirely formed on the bottom surface of the second substrate 51 having the black matrix 52.
However, the conventional method for manufacturing a liquid crystal display device as described above has problems as follows.
That is, a black matrix typically formed on a top substrate corresponds to a thin transistor formed on a bottom substrate in order to prevent light leakage at stepped parts due to color overlap. Therefore, formation of such a black matrix may cause an increase in production costs and light leakage due to misalignment during formation of the black matrix.